Abstract: An underground reservoir for storing water in alluvial deposits utilizes slurry walls keyed to an aquiclude beneath the reservoir to form a substantially impermeable water seal. A variety of water extraction/recharge apparatus provides for rapid and complete filling and draining of the underground reservoir. Such apparatus includes a plurality of wells distributed about the underground reservoir and a series of perforated pipes buried at a predetermined depth within the underground reservoir. A method of mitigating water table fluctuations resulting from the construction of an underground utilizes perforated pipes buried outside of the reservoir to transfer excess water from an uphill land parcel to a downhill land parcel that would otherwise experience a water shortage after construction of the underground reservoir.